Calorimetric study of blends of low density polyethylene (LDPE) and linear low density polyethylene (LLDPE) temperature rising elution fractionation (TREF) fractions

Preparative temperature rising elution fractionation (TREF) on commercial linear low density polyethylene (LLDPE) and low density polyethylene (LDPE) samples has been performed. The resulting fractions exhibited a bimodal and a unimodal distribution, respectively. Two LLDPE fractions of low (5 CH₃/1000 C) and high (21 CH₃/1000 C) short-chain branching content were solution-mixed with the LDPE central fraction (16 CH₃/1000 C). Indirect evidence based on differential scanning calorimetry results suggest that the fractions with similar branch contents are more miscible than those with dissimilar ones.     

Synthesis of a polyethylene-graft-polystyrene copolymer and its compatibilization for linear low density polyethylene/poly(phenylene oxide) blends

Based on unsteady diffusionkinetics, polyethylene(PE)-graft-polystyrene (PS) copolymers were designed and synthesized with a heterogeneous high yield titanium-based catalyst by copolymerization of ethylene with a PS-macromonomer using 1-hexene as a short main agent to promote the incorporation of the PS-macromonomer. The presence of 1-hexene facilitated the diffusion of the PS-macromonomer, giving rise to the significantly increased incorporation of the PS-macromonomer. Compatibilization of blends of linear low density polyethylene (LLDPE)/poly(phenylene oxide) (PPO) with the PE-g-PS copolymer were investigated using scanning electron microscopy (SEM) and dynamic mechanical analysis (DMA).     

Study on the miscibility of poly(2-hydroxyethyl methacrylate)/poly(styrene-co-acrylamide) blends by nonradiative energy transfer fluorescence spectroscopy

The miscibility of poly(2-hydroxyethyl methacrylate)/poly(styrene-co-acrylamide) (PHEMA/PSAm) blends was investigated by nonradiative energy transfer fluorescence spectroscopy using anthracene-labeled PHEMA and carbazole-labeled PSAm. The results show that the ratio of the carbazole and anthracene emission intensity, I_c/I_a, passes through a minimum at 47 to 57 mol-% of acrylamide in PSAm copolymers in the blends, indicating that a ‘miscible window’ exists. It also was found that the results are consistent with those obtained by differential scanning calorimetry. The driving force for the miscibility of PHEMA/PSAm may be explained with a mean-field theory.     

Phase structure and compatibility studies in poly(ethylene oxide)/poly(methyl methacrylate) blends

Morphology, crystallinity, and melting behaviour of solution-cast films of poly(ethylene oxide) (PEO)/poly(methyl methacrylate) (PMMA) blends were investigated as a function of composition, using optical and scanning electron microscopy, differential scanning calorimetry, and dilatometry. Up to a content of about 40 wt.-% of PMMA the blend films are completely filled with PEO spherulites, no separated domains of PMMA being observed. This observation suggests, in agreement with small angle X-ray scattering analysis, that for such compositions the PMMA molecules are incorporated in interlamellar regions of PEO spherulites. Blends with higher content of PMMA show islands of crystalline PEO within a matrix of PMMA; large amorphous regions coexist with not well developed PEO spherulites. Addition of PMMA strongly increases the nucleation density for PEO crystals. The solubility parameters δ of PEO and PMMA were calculated from specific volume values as a function of temperature. It was found that at temperatures higher than that of PEO melting the difference δ(PEO) - δ(PMMA) is very low suggesting that the two polymers are compatible in the melt. Compatibility of PEO and PMMA was also predicted by using a theoretical approach which allows to calculate the free energy of mixing as a function of composition and temperature.     

Spherulites primary nucleation in binary miscible blends of poly(ethylene oxide) with poly(methyl methacrylate)

The influence of composition on the primary nucleation process of poly(ethylene oxide) spherulites in the case of poly(ethylene oxide)/poly(methyl methacrylate) (PEO/PMMA) blends was investigated by optical microscopy and DSC. In the case of isothermally crystallized samples it was found that heterogeneous nucleation is depressed in the blends in comparison with plain PEO. This depression is caused by the dilution effect of PMMA. The self-seeding nucleation mode is also disturbed by the presence of PMMA in the blends. This is due to both the diluent effect and to the decrease of the PEO melting temperature. In the case of non-isothermally crystallized samples of PEO/PMMA blends an apparent increase of the nucleation density was observed. This observation is attributed to the fact that the growth rate is depressed in the blends: thus, at an earlier stage of crystallization, the volume in which the nuclei are free to develop (for a given crystallization condition) is larger.     

Stabilization of polystyrene/poly(methyl methacrylate) blends by in situ compatibilization with graft copolymers

Copolymers of styrene and ortho-vinylbenzaldehyde (o-VBA) are useful precursors to multicomponent polymer systems. Graft copolymers of poly(styrene-stat-o-VBA) can be produced by free-radical chain transfer to methyl methacrylate monomer to yield materials that display some potential as interfacial agents with binary blends of polystyrene and poly(methyl methacrylate). Significant improvements in ultimate tensile strengths and energy to rupture values have been witnessed for polystyrene/poly(methyl methacrylate) mixtures that contain various levels of the graft component. Scanning electron microscopy of representative fracture surfaces demonstrate a decrease in the particle size of the dispersed phase; it is suggested that this morphological factor contributes to the superior mechanical properties of these composites. In this regard, both the isolated and the crude graft copolymers are able to compatibilize, and thereby enhance, the material properties of polystyrene/poly(methyl methacrylate) blends.     

Synthesis of ethylene oxide/methyl methacrylate diblock copolymers by group transfer polymerization of methyl methacrylate with poly(ethylene oxide) macroinitiators

Ethylene oxide/methyl methacrylate diblock copolymers were prepared by group transfer polymerization (GTP) of methyl methacrylate (MMA) with poly(ethylene oxide) macroinitiators ( 1 ). The macroinitiator containing silyl ketene acetal end-groups was synthesized by hydrosilation of poly(ethylene glycol) monomethacrylate which was previously prepared by esterification of poly(ethylene glycol) monomethyl ether (PEO OH). All synthesized macroinitiators initiated the GTP of MMA in tetrahydrofuran with tetrabutylammonium cyanide or tris(dimethylamino)sulfonium difluoride as catalysts. The polymerization proceeds rapidly at room temperature to quantitative yield. The block copolymers are contaminated with small fractions of PEO homopolymer as a result of uncomplete macroinitiator synthesis. Unreacted PEO—OH, which is present in very small amounts, reduces the concentration of active centers. Therefore, the degree of polymerization is not in good agreement with the theoretical value calculated for a living polymerization taking the monomer to macroinitiator ratio. However, after extraction of the PEO homopolymer, the block copolymers show a narrow molecular weight distribution. Induction periods in time-conversion curves obtained with 1 as initiator vanished nearly, when the reaction was started with the addition product of 1 and MMA. This is an indication, that in the system under investigation a slow initiation step between 1 and the first MMA unit precedes the propagation reaction. Tacticity measurements yield about 56% syndiotactic, 40% heterotactic and 4% isotactic triads for PMMA in the block copolymers.     

Block copolymers by sequential group transfer polymerization: poly(methyl methacrylate)-block-poly(2-ethylhexyl acrylate) and poly(methyl methacrylate)-block-poly(tert-butyl acrylate)

Poly(methyl methacrylate)-block-poly(2-ethylhexyl acrylate) (PMMA-block-PEHA) and poly(methyl methacrylate)-block-poly(tert-butyl acrylate) with a methacrylate/acrylate unit ratio of 1:1 and 1:3, 16000 < M _n < 44000 and 1,9 < M _w/M _n < 2,5, were prepared by sequential group transfer polymerization using (1-methoxy-2-methyl-1-propenyloxy)trimethylsilane as initiator and tetrabutylammonium fluoride monohydrate as a catalyst in tetrahydrofuran at ?30°C, PMMA being the first block. The increase in M _n during the successive addition of monomers is linearly dependent on the (co)polymer yield and size-exclusion chromatography (SEC) curves are shifted towards higher molecular weights in comparison with PMMA macroinitiators. The block structure of the copolymers was also proven by extraction experiments. The presence of homopolymers in the copolymers was not detected. When the former copolymer is prepared in a reverse way (PEHA segment being the first), the MMA polymerization ceases at ≈ 43–45% conversion.     

The stereoregularity of poly(methyl methacrylate)s formed in the presence of highly syndiotactic poly(methyl methacrylate)s

Methyl methacrylate (MMA) was radically polymerized in the presence of highly syndiotactic deuterated or undeuterated poly(methyl methacrylate)s. In contrast to observations found in the literature no increase in isotacticity was observed. The stereoregularity of the polymers formed in the presence of an undeuterated PMMA as matrix was found by ¹H NMR spectroscopy to be about the same as that in polymers prepared without any matrix, except the polymers formed in the early stage of polymerization which show a little lower syndiotacticity. Determinations of triad tacticities of polymers formed in the presence of deuterated PMMA, gave values of syndiotacticity a little higher than in the case of undeuterated matrixes. The polymer formed in the graft copolymerization of MMA onto pre-irradiated syndiotactic PMMA was fractionated by GPC. The high molecular weight fraction showed a lower syndiotacticity than that of lower molecular weight.     

Control of cell adhesion on poly(methyl methacrylate)

Keratoprostheses have been constructed from a wide variety of transparent materials, including poly(methyl methacrylate) (PMMA). However, the success of keratoprosthesis has been plagued by numerous shortcomings that include the weakening of the implant-host interface due to weak cell adhesion and opaque fibrous membrane formation over the inner surface of the implant due to fibroblast attachment. An effective solution requires a surface modification that would selectively allow enhanced cell attachment at the implant-host interface and reduced cell attachment over the interior surface of the implant. Here, we have developed a novel and simple peptide conjugation scheme to modify PMMA surfaces, which allowed for region-specific control of cell adhesion. This method uses di-amino-PEG, which can be grafted onto PMMA using hydrolysis or aminolysis method. PEG can resist cell adhesion and protein adsorption. The functionalization of grafted di-amino-PEG molecules with RGD peptide not only restored cell adhesion to the surfaces, but also enhanced cell attachment and spreading as compared to untreated PMMA surfaces. Long-term cell migration and micropatterning studies clearly indicated that PEG-PMMA surfaces with and without RGD conjugation can be used to differentiate cell adhesion and control cell attachment spatially on PMMA, which will have potential applications in the modification of keratoprostheses.     

Synthesis of a stereoblock poly(methyl methacrylate)

The sequential block copolymerization of diphenylmethyl methacrylate (DMA) and trityl methacrylate (TrMA), initiated by diphenyl methyl lithium in THF at -78°C followed by hydrolysis and methylation with diazomethane, leads to the formation of an isotacticsyndiotactic stereoblock poly(methylmethacrylate) (PMMA) of narrow molecular weight and block length distribution. Complete conversion of DMA is shown to be essential, since the presence of small quantities of this monomer during the polymerization of TrMA significantly lowers the isotactic content of this block. Polymerization times of at least 12 h were found to be necessary for complete polymerization of the TrMA.     

Carbon-13 relaxation study of crosslinked gels of poly(methyl methacrylate)

The motion of covalently crosslinked gels of poly(methyl methacrylate) (PMMA) swollen by benzene-d₆ was studied by means of ¹³C NMR spectroscopy. The dependence of the segmental motion of PMMA on stereosequence was not affected by the presence of the covalently crosslinked part in the chain; isotactic sequence was more mobile than syndiotactic sequence. The spin-lattice relaxation times, T₁'s, for all the ¹³C nuclei of PMMA swollen with benzene (benzene content, 90 wt.-%, 0,01 mole-% of crosslinking) were considerably longer than those observed for the linear PMMA (benzene content, 75 wt.-%). Moreover, all the T₁ values decreased with increasing of the degree of crosslinking, regardless of the crosslinking agent and reached to the values of T₁ observed for uncrosslinked PMMA or to slightly larger values when the content of crosslinking agent was 1 mole-%. However, the nuclear Overhauser enhancement data were essentially independent of the degree of crosslinking within experimental error.     

The miscibility and phase separation in binary blends of poly(vinyl chloride) with methyl methacrylate/acrylonitrile copolymers

The miscibility of poly(vinyl chloride) with poly(methyl methacrylate-stat-acrylonitrile) has been studied. Poly(vinyl chloride) is miscible with methyl methacrylate/acrylonitrile copolymers having acrylonitrile contents between 2,0 and 17 wt.-%. All calculated binary segmental interaction parameters were positive, suggesting immiscibility of the corresponding binary homopolymer blends.     

NMR study of aggregation of heterotactic-like poly(methyl methacrylate) in solution

The aggregation of heterotactic-like poly(methyl methacrylate) (PMMA) in some solutions was proved by NMR spectroscopy. It was found that the fraction of aggregated units depends on solvent, solution concentration and temperature; the aggregates are completely decomposed at 45°C. Aggregation occurs by intermolecular interactions of m-diads with rrr-tetrads, i. e. by interactions of the stereocomplex type. The formation of a stable aggregated structure is probably a consequence of the microstructure of the heterotactic-like PMMA chain, with relatively long sequences of regularly repeating mrrr pentads or mrrr hexads. The aggregation in mixtures of heterotactic-like PMMA solutions with isotactic or syndiotactic PMMA solutions was also studied.     

Molecular orientation relaxation in binary blends of poly(methyl methacrylate) by rheo-optical Fourier-transform infrared spectroscopy

Rheo-optical Fourier-transform infrared spectroscopy, a technique combining simultaneous mechanical measurements and infrared dichroism spectroscopy, has been employed in order to investigate the molecular chain orientation and orientation relaxation behaviour in uniaxially stretched films of binary blends of long perdeuterated and short undeuterated chains of linear poly(methyl methacrylate) (PMMA). The solution cast film samples have been stretched at constant strain rate up to 100% elongation and subsequently allowed to relax at constant strain at various temperatures above the glass transition temperature. The results reported here show that for the short-chain component a threshold value of molecular weight exists below which a drastic decrease of the long-and short-chain orientation and a change in the relaxation behaviour of the short chains are observed.     

The glass transition temperatures of homogeneous blends of polystyrene,poly(methyl methacrylate), and copolymers of styrene and methyl methacrylate

The glass transition temperatures T_g of homogeneous binary blends of the homo- and the statistical copolymers of the system poly(styrene-co-methyl methacrylate) were studied. Some of the blends, which are in the equilibrium phase separated, were forced into homogeneity. T_g (?) of the homopolymer blends (PS/PMMA)? follows the Fox equation, while T_g (x) of the pure copolymers P(S_xMMA_{1 ? x}) exhibits a minimum. This minimum can be effectively removed by blending the copolymers with small fractions (? ≈ 0,2) of one of the two homopolymers or a differently composed copolymer P(S_yMMA_{1 ? y}).     

Microbial adherence on poly(methyl methacrylate) (PMMA) surfaces.

Infection remains a major complication following the use of implanted biomaterials. Often these infections are caused by low-virulence organisms or by a mixture of organisms (polymicrobial). In this study two methods were used to quantitate the bacteria which had adhered to poly(methyl methacrylate) (PMMA) samples. The bacteria were eluted from the sample using an ultrasonic cleaner. The number eluted was then counted by colony counts, which determines viable organisms and by particle counting which counts both viable and nonviable organisms. A known adherent strain of Staphylococcus epidermidis and a strain of Proteus mirabilis were used. In general the adherence of S. epidermidis was greater than that of Proteus. When the two organisms were used together, there was an alteration in the adherence pattern which generally increased the adherence of Proteus and had no effect or decreased the adherence of S. epidermidis. The use of both quantitation techniques provided important information on the adherence of organisms to PMMA to which gentamicin had been added. It was evident that organisms did adhere to the PMMA plus gentamicin samples but were not viable when eluted. The amount of adherence to PMMA plus gentamicin was similar to that of PMMA alone at 30 min but was markedly decreased at 24 h. There was a considerable dead biofilm mass on the PMMA plus gentamicin samples which might be a significant promoter of late infections by providing a surface attractive to other strains of bacteria.     

Viscosities of dilute solutions of poly (methyl methacrylate) and poly (ethyl methacrylate) in organic solvents

Poly(methyl methacrylate) and poly(ethyl methacrylate) prepared by a benzoylperoxide catalysed polymerization process were fractionated. The variation of the intrinsic viscosity and HUGGINS ' constant with temperature, molecular weight and solvent was studied. From the viscosity data the thermodynamic interaction parameters χ, ψ, k etc., and the solubility parameter of the polymers were evaluated and discussed.     

Compatibility of poly(ethylene oxide) and poly(methyl methacrylate) chains in solutions of their blends,diblock and triblock copolymers

Solvent activity data were obtained from vapour pressure measurements at 50°C, 70°C, 100°C and reduced to χ-functions in systems consisting of benzene or toluene on the one hand and blends or block copolymers of poly(ethylene oxide) and poly(methyl methacrylate) on the other. The χ_BC-values are negative for all polymer subsystems, but miscibility on a segmental scale is somewhat different for block copolymers and blends. χ_BC depends on the concentration of solvent and is mainly influenced by the Δχ-effect in solution and correlated with the syndiotacticity of the PMMA chains. There is only a small influence of temperature and blend composition or block ratio.